In recent years, the production of ethanol or raw materials for chemical products from cellulose, which is a regenerable and carbon neutral resource, has been in strong demand in response to problems such as fossil resource depletion and global warming.
Cellulose is contained in abundance in herbaceous plants and woody plants, which are collectively referred to as cellulosic biomass. The cell walls of cellulosic biomass are mainly composed of cellulose, hemicellulose, and lignin. Cellulose is a linear polysaccharide comprising glucose molecules joined by β-1,4 linkages. Hemicellulose is a polysaccharide such as xyloglucan, xylan, or mannan. Lignin is an aromatic macromolecular compound with a complicated structure, intertwined with cellulose and hemicellulose within cell walls to form a three-dimensional mesh structure.
The production of ethanol or raw materials for chemical products from cellulosic biomass requires a step referred to as “saccharification” by which cellulosic biomass is degraded into monosaccharides that can be fermented by microorganisms. Examples of typical saccharification processes include acid treatment and enzyme treatment. Acid treatment involves a large amount of waste water, imposing a great environmental burden. Hence, enzyme treatment, which involves performing a reaction under moderate conditions using cellulase, is currently the mainstream treatment under development.
Cellulase is a generic name applied to cellulose-hydrolyzing enzymes, which are classified into three types based on substrate specificity differences: cellobiohydrolase, endoglucanase, and β-glucosidase. They are believed to act in concert so that cellulose is hydrolyzed.
When cellulosic biomass is saccharified using cellulase, the activity of cellulase is inhibited by various factors such as substrate inhibition, product inhibition, and non-specific adsorption. Furthermore, it is known that the activity of cellulases such as endoglucanase is inhibited by lignin-derived aromatic compounds (R. M. Vohra et al., Biotechnol. Bioeng., 22, 1497-1500 (1980), S. S. Paul et al., Lett. Appl. Microbiol., 36, 377-381 (2003) and E. Ximense et al., Enzym Microb Tech., 46, 170-176 (2010)). However, the mechanisms of inhibition remain unknown.
The enzymes produced by thermophilic bacteria or hyperthermophilic bacteria are highly stable and thus can retain their activity even under high-temperature conditions for long periods of time. Hence, the application thereof as industrial enzymes has been examined. Cellulases produced by cellulose-degrading thermophilic bacteria or hyperthermophilic bacteria have also been studied. It has been revealed that most of the cellulase genes of these bacteria encode endoglucanases.